The basic techniques of plasma generation using microwaves are well known. See e.g. "Semiconductor Plasma Processing Technique" (Sugano, published by Sangyo Tosho, page 139). Electric fields created by microwaves and an external magnetic field act together in a discharge tube to create a plasma which is used for treating e.g. semiconductor wafers.
As semiconductor technology advances, the size of semiconductor wafers increases. To plasma-process such wafers, the propagating waveguide for the microwaves must also be enlarged, so that a larger area of plasma can be created. However, the increase in waveguide area brings with it certain problems, which reduce the quality and consistency of the plasma processing currently available with large-section waveguides.